Low temperature hypobaric storage of metabolically active matter

ABSTRACT

The preservation of metabolically active matter such as fruit, vegetables, meat, fowl, shrimp, fish, other food, cut flowers, cuttings, foliage plants and the like is disclosed, characterized by storage at controlled and correlated conditions of hypobaric pressure, temperature, humidity, air circulation and air exchange. A non-deleterious gas such as air is humidified by contacting it with heated water from a supply, and then the humid air is passed through, and when advisable, recirculated and/or rehumidified within a storage chamber containing the metabolically active matter. The relative humidity is maintained within the range of about 80 to 100% and the pressure is maintained continuously or intermittently at a selected value at least slightly higher than the vapor pressure of the water in the stored commodity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my pending applicationfiled in my name, Stanley P. Burg, on 20th April 1972, Ser. No. 245,886,entitled "Low Temperature Hypobaric Storage of Non-Mineral Matter"nowabandoned. Certain aspects of this application are related to the jointU.S. patent applications filed in my name, Stanley P. Burg, and WilliamJ. Hentschel, entitled "Low Pressure Storage of Metabolically ActiveMaterial with Open Cycle Refrigeration", filed Jan. 5, 1972, Ser. No.215,688, now issued in U.S. Pat. NO. 3,810,508, and a division thereoffiled March 26, 1974, Ser. No. 454,825 now U.S. Pat. No. 3,913,661. Thisapplication is also related to U.S. Pat. No. 3,333,967 and my copendingapplication for reissue thereof filed Mar. 26, 1973, Ser. NO. 345,182.

BACKGROUND OF THE INVENTION

In my prior U.S. Pat. No. 3,333,967, a method is disclosed forpreserving mature but less than fully ripe fruit which produce ethyleneand are ripened thereby, using hypobaric conditions of about 100 to 400millimeters of mercury (mm Hg) absolute pressure in nearlywater-saturated, moving air to facilitate the diffusive escape ofethylene from the commodity without loss of water therefrom. This methodwas first discovered to give useful results on a laboratory scale, andlater under favorable commercial conditions on a larger scale, withnon-ripe, mature fruit such as avocados, limes, and especially bananas.Upon later study and research I discovered that hypobaric, i.e. lowabsolute pressure, storage is valuable for reasons in addition topromoting the diffusive escape of ethylene from stored commodities, andthat consequently the method is applicable to a wide variety ofmetabolically active matter other than that which is influenced byethylene. In some instances, for example meat, it presently appears thatthe efficacy of the method is due to a reduction in oxygen partialpressure which attends the pressure reduction, as well as to enhanceddiffusive escape of volatile off-odors, but with other commodities moreis involved. For example, the foliage of chrysanthemums produce largequantities of ethylene and is not affected by the gas, yet it respondswell to hypobaric but not low-oxygen, atmospheric pressure storage. Ialso learned how to overcome several problems and difficulties which hadrestricted the use of the hypobaric method to pressures higher than the100 mm Hg. limit described and claimed in my U.S. Pat. NO. 3,333,967.

It is known as in Bonomi's Br. Patent No. 822,904 that at atmosphericpressure, many forms of metabolically active matter ferment and arespoiled by the accumulated waste products of anaerobic respiration ifcontinuously exposed to less than 3% oxygen. This happens to beequivalent to the oxygen partial pressure in air at 100 mm Hg. absolutepressure. Bonomi teaches only superatmospheric pressures between about832-905 mm Hg. absolute and subatmospheric pressures between about687-650 mm Hg. absolute: quite remote from my hypobaric method.

Heat exchange is so limited by the decreased heat capacity of air atpressures lower than 100 mm Hg. that it is not possible to cool acommodity and maintain it at a uniform temperature in dry air underthese conditions. Indeed, at some low pressure which is unpredictablebecause it is determined in part by the geometry of the apparatus, theDewar effect sets in and prevents all conductive heat transfer. I havediscovered that the Dewar effect does not influence heat transfer in acommercially sized hypobaric trailer at pressures as low as 8 mm Hg.,that conductive heat exchange can be kept at a satisfactory value atpressures lower than 100 mm Hg. by saturating the atmosphere with watervapor, that certain of the deleterious effects of too low an oxygenpartial pressure can be obviated with advantage by periodically cyclingthe pressure back to atmospheric, and that fermentative waste products,being volatile compounds can be in part removed under hypobaricconditions.

These discoveries and improvements have enabled me to use the hypobaricprocess at pressures lower than 100 mm Hg., and thereby to increase itsefficacy with certain commodities and extend its utility to othercommodities. I have found that at pressures lower than 100 mm Hg., andespecially below 50 mm Hg., even though the atmosphere is kept fullysaturated with water vapor, commodity desiccation may occur because thecommodity respires and thereby is slightly warmer than the surroundingatmosphere. The higher temperature causes the vapor pressure of water inthe commodities to be greater than that in the surrounding air. At a lowpressure the rate of diffusion of water vapor is so enhanced that theslight tendency for water movement from the commodity to the atmosphere,created by the vapor pressure differential, is greatly magnified. Ilearned that the use of certain water retentive plastic wraps sufficesto prevent desiccation under these conditions, but the atmosphere stillmust be kept saturated with water, for the rate of passage of watervapor through the wraps is enhanced considerably when the pressure islowered, thus rendering these moisture barriers far less efficient thanif they are used at atmospheric pressure.

Another problem which becomes increasingly important as the pressure isreduced, especially below about 100 mm Hg., is evaporative cooling ofthe humidifying water. Upon enlarging the size of the hypobaric storagechamber to large commercial proportions, I learned that, because of thisevaporative cooling effect, the said method and means disclosed in myU.S. Pat. No. 3,333,967 under various unfavorable conditions could failto provide or maintain the high relative humidity that presently seemsimportant for successful operation without a much reduced airthrough-flow.

I have now discovered how to use and profit by, and avoid deleteriousconsequences of, the refrigeration effect incident to free air expansionand water evaporation when air is bubbled through a body of humidifyingwater which is relatively smaller in relation to the whole storage spacethan was the body of water in relation to the size of a conventionallaboratory vacuum vessel. While this cooling, often or sometimes, isdesirable to lessen the work of, or eliminate other ways and means ofcooling the chamber, refrigerating by evaporation of water runs counterto the objective of creating and maintaining high humidity. As the watercools, its vapor pressure is lowered and it tends to add progressivelyless water vapor to the incoming air so that the relative humidity inthe chamber is reduced. In extreme cases, such as storage conditionsnear 0°C, the water of the humidifier can freeze because of evaporativecooling.

SUMMARY OF THE INVENTION

Maintaining the temperature of the humidifying water and make-up air inan advantageous relation to that of the air in the hypobaric chamber inorder to provide a constant high relative humidity within the chamber isone of the objects of my invention. By pre-conditioning the make-up airto a temperature close to that in the hypobaric chamber, and heating thewater to a higher temperature, the control of humidity within thechamber is made independent of ambient conditions and the evaporativecooling effect.

I have discovered that a relatively broad spectrum of correlatedhypobaric pressures and low temperatures at high relative humidity isoperational in preserving metabolically active matter, different classesof which vary markedly in their required or permissible storageconditions for best results. I find that pressure and temperature ofteninteract in such a way that each factor influences the lowest and/orhighest permissible value of the other which is needed to create themost beneficial storage condition. It presently appears that the bestcombination of these and other factors can be determined only bycarefully ccontrolled tests.

A primary object of the invention is to provide optimal, correlatedconditions of temperature, pressure, humidity and air movement forstoring different categories, respectively of metabolically activematter. In the present method, a preferred correlation dependent uponthe nature of the stored matter is established and maintained betweenranges and continuities of temperature, air flow, air recirculation,hypobaric pressure and humidity. My reference herein to air andhumidified air will comprehend equivalent non-deleterious gases andhumidified gases unless otherwise noted.

An object of my invention is to improve upon my, and all other, priorpatents, and extend the field of usefulness and the benefits andadvantages of humid, subatmospheric cold storage, to the preservation ofmetabolically active matter, produce and commodities beyond thecontemplation and facility of the teaching of the prior art.

Other objects, improvements and advantages will appear herein, referencebeing made to illustrative examples in the following pages, and to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing:

FIG. 1 is in part a schematic flow diagram and in part a diagrammaticcrepresentation of one form of chamber or container and apparatusembodying and/or for practicing my invention; and

FIG. 2 is a flow diagram and representation like FIG. 1 of another formof chamber or container, and apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, there are at least five factors or conditions mutuallyinfluencing the storage of metabolically active matter (hereinafter"MAM") in my hypobaric system, namely:

1. The kind or category of the stored MAM.

2. air pressure in the storage chamber as may be maintained continuouslyor intermittently.

3. Air temperature in the storage chamber.

4. Relative humidity of the air in the storage chamber.

5. Rate of air flow into and out of the storage chamber, andcirculation, recirculation and/or rehumidification of air therein.

Additional factors that in some instances influence storage at hypobaricpressures principally below 100 mm Hg. include the use of waterretentive means to wrap the commodity, and the periodic or occasionalcycling of pressure between hypobaric and atmospheric.

There is a relatively broad operational range of cool-to-coldtemperatures, hypobaric pressures, high humidity and air movement ratesin which most MAM can be stored advantageously as compared withatmospheric conditions and those described in my prior U.S. Pat. No.3,333,967. Within these basic ranges, the nature of the commodityconstituting the MAM determines optimum temperature, pressure, air flowand air recirculation conditions which can differ widely for differentkinds of commodities. However, in all cases it presently appears that adesired high humidity level should be maintained to protect thecommodity, regardless of other conditions. Although this humidity levelpreferably should be close to 100 %, in cases such as storing limes, forexample, a slightly lower humidity is often preferred to discourage thegrowth of molds. Gross air movement to and from the storage chamberremoves deleterious gases which tend to be given off by and from thestored matter, but increasing the rate of through-flow of air tends tobear adversely on the maintenance of desirably high humidity because ofthe aforementioned evaporative cooling effect. Internal recirculation ofair within the chamber facilitates rehumidification if the air isrecycled through the humidification means, and improves heat transfer,insuring that all the stored contents of the chamber are bathed in aboutthe same atmosphere and kept at a uniform temperature. Recirculation isparticularly important during the initial stages of storage when fieldheat is being removed from the stored contents. On the other hand,increasing the rate of recirculation of air within the chamber tends todry the organic matter, as described below, so that optimal storageusually is realized only at a specific range of air recirculation and/orthrough-put.

More particularly, my present improvement is applicable to a wide rangeof commodities including, in different classes or categories, not onlymature but less than fully ripe fruit, but also ripe fruit, vegetables,cut flowers, potted plants, meat products, especially red meats such asbeef and pork, and fowl such as chickens, shrimp, fish, vegetables andvegetative matter such as rooted and non-rooted cuttings, and stillother metabolically active plant materials such as bulbs, corms, seeds,nuts, tubers, dried alfalfa pellets and the like. Such matter ofdifferent kinds and classes respectively may, as I have presentlylearned, be stored with advantage at pressures between about 4 mm Hg.and 400 mm Hg. absolute and at temperatures between about minus 2° toabout plus 15°C with exceptions for particular kinds of matter andcircumstances wherein the said upper limits of one or the other,temperature or pressure, may be exceeded advantageously; all taken withmy preferred conditions of humidity and air movement. The minimumstorage pressure, unless it is determined by oxygen availability, can bealmost as low as the vapor pressure of water at the temperature ofstorage. If the storage pressure were to reach the vapor pressure ofwater at the storage temperature, the water in the product andhumidifier would boil and my method, as presently understood, would beimpaired, if not rendered inoperable. Optimal correlated ranges oftemperature, pressure, humidity and air movement within the hereinstated ranges for specific commodities and/or classes of commodities arenot readily predictable without careful experiment and research,preferably on both laboratory and commercial scales.

It is presently believed to be desirable to store commodities at thelowest temperature which does not cause chilling damage. Pressure mayalso play a role with respect to operable temperature because, at leastin some cases, cold damage seems to be caused by accumulation ofvolatile metabolic products such as farnescece, alcohol, oracetaldehyde. Since hypobaric pressures tend to remove these products,they sometimes alleviate, delay or reduce the symptoms of cold damageand permit storage at temperatures which otherwise might impart colddamage. In other cases, because the oxygen partial pressure is reducedunder hypobaric conditions, thereby favoring the fermentative productionof many of the same substances which accumulate in response to colddamage, there is an increase of adverse cold temperature reactions suchas peel pitting, and browning in certain fruits if the storage pressureis too low, whereas at high pressures the chilling effect still may bealleviated with the same commodity. For example, unwaxed Tahiti Persianlimes keep their green color best if they are stored at 80 mm Hg.pressure and 9°C, but they experience chilling damage at that pressureand a temperature of 7°C. At 7°C they are best stored at 150 mm Hg., inwhich case they keep their green color without rind breakdown for manymonths. However, at that pressure they develop rind breakdown if thetemperature is lowered to 5°C. In order to prevent rind breakdown at5°C, the pressure must be elevated to 250 to 300 mm Hg., in which casethe treated fruits experience less cold damage than fruits stored atatmospheric pressure and the same temperature. Similar results have beenobtained with Marsh and Ruby red grapefruit. However, with bananas attemperatures lower than 12°C, cold damage is not enhanced but rather isalleviated as the pressure is reduced from 250 to 40 mm Hg. For thesereasons it has not yet been possible for me to predict or extrapolatefrom studies of cold tolerance of one commodity under hypobaricconditions all the aspects of behavior that fact commodity willhypobaric conditions

Similarly, I have found it difficult, if not impossible to predict orextrapolate directly from available studies of the chilling effect atreduced partial pressures of oxygen at standard conditions, what theeffect, or corresponding effect, will be on storage at subatmosphericpressures. In part the lack of agreement between studies at atmosphericand subatmospheric pressure is due to the fact that under hypobaricconditions the rate of oxygen consumption appears to be different eventhough the oxygen partial pressure is the same. In both casesrespiration and heat evolution are greatly reduced when the oxygenavailability is restricted, but progressively the tissues stored underhypobaric conditions continue to decrease in respiration rate,ultimately reaching far lower values than comparable samples kept at theanother oxygen partial exhibit. at atmospheric pressure. Moreover, mostof the reduced-oxygen, atmospheric pressure studies in the prior artwere carried out in air-tight or substantially air-tight containerswherein ethylene and respiratory carbon dioxide were permitted toaccumulate to relatively high concentrations and to interactsubsequently with each other and the oxygen in such a manner so as torender it difficult if not impossible to predict or extrapolate fromthese studies the behavior of commodities in renewing hypobaricatmospheres which remove carbon dioxide and ethylene.

I have also found it difficult if not impossible to predict orextrapolate from hypobaric studies in the pressure range of 100 to 400mm Hg. whether the method is applicable to the pressure range of 4 to100 mm Hg. (all pressures mentioned herein are absolute unless otherwisespecified), and whether the lower range will be more beneficial than thehigher range for various commodities. The reduction in oxygenavailability by decreasing the pressure from 100 mm Hg. to about 4 mmHg. causes two successive shifts in the type of microbial flora that maydevelop in the commodity and lead to decay and to the growth ofpathogens that would render the commodity unfit for consumption. At 100mm Hg. there is sufficient oxygen to support the growth of aerobicbacteria and molds but too much oxygen to support anaerobic bacteria,facultative anaerobic bacteria and microaerophylic bacteria. At about 40to 50 mm Hg., the oxygen content reaches a level which inhibits thegrowth of aerobic bacteria and molds but promotes the growth offacultative anaerobes, microaerophylic organisms, and certain yeasts. Ifthe pressure is reduced so that it approaches the vapor pressure ofwater at the storage temperature, anaerobic organisms develop and thegrowth of all other types is inhibited. The fact that numerous essentialmetabolites should boil from the tissue at -2° to 15°C in the 4 to 100mm Hg range whereas few if any should boil in the 100 to 400 mm Hg.range further complicates the use of hypobaric data taken in the 100 to400 mm Hg. range.

At a relative humidity of about, and above, 80% and at given storagetemperatures as shown in examples below, MAM which I have noticed to bebest stored in the 100 to 400 mm Hg. range tends to be mature but lessthan fully ripe bananas and limes as specified in my prior U.S. Pat. No.3,333,967. Under certain conditions, as when the temperature is 10°Cwith limes, or less than 12°C with bananas, pressures lower than 100 mmHg. may give slightly improved results. Departing from that patent Ihave now learned that fully ripe fruit, such as strawberries, cherries,grapefruits, tomatoes and blueberries, are well stored in the 80-400 mmHg. range but at much lower temperatures. The storage of cut flowers isimproved by pressures at and above 100 mm Hg., but markedly so at 25 to70 mm Hg. While apples and pears may be stored in a pressure range ofabout 100 to 150 mm Hg., markedly improved results are obtained atpressures in the range of about 40 to 80 mm Hg. In general most non-ripemature fruits are best stored at 50 to 80 mm Hg. Vegetative materialssuch as cuttings, rooted cuttings, and potted plants which I haveexamined store best with respect to pressure within the 40 to 80 mm Hg.range. Most vegetables except lettuce store best at a range within 50 to80 mm Hg. Red meats, fish, poultry, and shrimp store best in thepressure range of 8 to 15 mm Hg. although the poultry and shrimp alsobenefit from slightly higher pressures.

In plant tissues a shift from the normal aerobic respiration toanaerobic fermentative respiration yielding carbon dioxide, acetaldehydeand alcohol, occurs in the pressure range of 4 to 100 mm Hg., just as itdoes at atmospheric pressure when the oxygen partial pressure is reducedto a comparable value, e.g. less than about 3%. If the oxygen level ismaintained low enough for an extended period of time, the toxic endproducts of fermentation may accumulate in spite of the enhancedtendency for their diffusive escape, and eventually the commodityundergoes internal breakdown. However, the rate of diffusive oxygenentry into tissue under hypobaric conditions is so much more rapid thatthe oxygen concentration within the fruit is kept much closer in valueto the external oxygen concentration than is the case at atmosphericpressure. Consequently fermentation occurs at a higher external oxygenpartial pressure under atmospheric conditions.

In order to alleviate or prevent undesirable adaptation to the lowoxygen environment and the resulting internal breakdown I haveperiodically cycled the absolute pressure from the selected runningpressure up to atmospheric pressure to expose the commodity to a normalatmospheric oxygen level for the selected length of the cycle. Withchrysanthemum cuttings the pressure was allowed to rise from 40 mm Hg.to atmospheric pressure for various selected periods of 2, 4, 6, 8, 10,12 or 14 consecutive hours respectively, out of 24 hours each day. Avery slight diminution in quality was noted with the 14 hour cycle butthose cycled for 2 to 8 hours remained greener than cuttings kept undera continuous hypobaric pressure of 40 mm Hg. With mature green tomatoesat 13°C for 8 weeks, the pressure was allowed to rise from 80 mm Hg. toatmospheric pressure for 2, 4, 6 or 8 hours each day. Best results wereobtained with a 4-hour cycle, which further reduced the rate of ripeningand greatly decreased the incidence of decay compared to fruits kept ata continuous pressure of 80 mm Hg. for the same period of time.

Intermittent application of 8 hour periods, for example, of atmosphericpressure also permits an access to the storage chamber for introductionto and/or removal of to-be-stored or stored produce or other MAM to andfrom the chamber.

Classification of the various and numerous kinds and varieties of MAM inrespect to the beneficience of their response to my presently preferredconditions of and for storage and preservation thereof within myimproved method and means, will suggest itself from the preceding pagesand the following numbered "Examples" and lettered "Tables". In theseexamples and tables, it is to be assumed that (1) the relative humiditywas created and maintained above 80% and as high as practicable anddesirable for the preservation and storage of the particular matter orproduce as variously taught herein, (2) through-put of air in terms ofvolumes of the storage chamber per hour did not fall below an efficientand desirable minimum according to the precepts hereof, and (3) the rateof internal circulation of the humid atmosphere of the storage chambersufficed to bathe all the stored contents of the chamber adequately andalso to humidify or rehumidify said atmosphere to create and/or maintainthe desired and most appropriate relative humidity in said chamber inview of the kind of matter or produce being preserved under theaccompanying conditions of temperature and subatmospheric pressure whichare specified in the several Examples and Tables.

The following examples are intended to illustrate my invention and arenot intended to limit or impair the scope of the claims. Where referenceis made below to "cold storage" as such, it is meant that such storageis old and conventional at atmospheric pressure, not LPS hypobaric,humid storage according to my present invention.

EXAMPLE 1

McIntosh, Red delicious, Golden delicious and Jonathan apples werestored at 60 mm Hg. and at 150 mm Hg., each batch at minus 1° to 2°C.Under normal cold storage conditions at these temperatures the differentvarieties may be kept for 2 to 4 months, except that in the case ofMcIntosh apples, chilling damage would be expected. After 6 months'storage at 60 mm Hg., all varieties still retained their initialfirmness, color, flavor and shelf-life, whereas at 150 mm Hg. they haddeveloped considerable aroma and were approaching full ripeness. After 8months, fruits stored at 60 mm Hg. still retained at-harvest appearance,and had acquired a shelf-life after removal from storage which wasconsiderably longer than that at harvest. Fruits stored at 150 mm Hg.were completely ripe at this time and their shelf-life foreshortened. At60 mm Hg. the storage life of McIntosh apples is limited to about 6months, at which time internal breakdown occurs. However, if the fruitis transferred to or subjected to cold storage (at atmospheric pressureas defined above) prior to that time, it can be kept for an even longerduration without internal breakdown, and subsequently ripens normally.In a companion experiment performed at 6°C, similar results wereobtained for the apples except that the storage life at each pressurecondition was vastly reduced compared to that at minus 1° to 2°C.

EXAMPLE 2

Bartlett, Clapp and Commice pears were stored at 60 mm Hg. and also at150 mm Hg. and at minus 1° to plus 1°C. Under normal cold storage atthese temperatures the pears may be preserved for one-and-a-half to 3months. At 150 mm Hg. the storage was improved, but at 60 mm Hg. thepears kept for 4 to 6 months in satisfactory condition. Upon subsequentremoval from storage, the pears ripened properly with no internalbrowning and had a normal shelf-life. In a companion experiment usingClapp pears, a pressure of 40 mm Hg. proved to be superior to 60 mm Hg.in prolonging storage life. In another like experiment, Bartlett pearswere stored at 6°C and separately at 40 mm Hg. and at 60 mm Hg. Underthese conditions they responded to hypobaric storage as before but ateach pressure their storage life was shorter than at minus 1° to plus1°C.

EXAMPLE 3

Cut roses, carnations, gladiolas, snapdragons, chrysanthemum and asterblossoms were stored at pressures ranging from 40 to 150 mm Hg. and at0° to 3°C. Under normal cold storage conditions in the dry state, theflowers faded within 1 to 2 weeks even when wrapped with perforatedpolyethylene film. Under hypobaric conditions, relatively small butsignificant benefit was realized at pressures of 100 to 150 mm Hg., butat 40 to 80 mm Hg. even without plastic wrap, the flowers were preservedin excellent condition for 4 to 9 weeks and still retained nearly theirinitial vase-life. The blooms respond to commercial flowerpreservatives. Storage life of flowers under hypobaric conditions isfurther improved by using plastic wraps. Snapdragons and gladiolas cannot be stored in a horizontal position at atmospheric pressure and 0° to3°C for within a few days they develop geotropic curvatures. At the sametemperature but under hypobaric conditions they can be storedhorizontally for several weeks without curving.

EXAMPLE 4

Potted chrysanthemum plants, vars. Neptune, Golden Anne, Delaware andBright Golden Anne, were stored at pressures ranging from 40 to 150 mmHg. and at 0° to 4°C. The plants were selected to have flower buds atvarious stages of opening. Under normal cold storage conditions, theshelf-life of the blooms after the plants are removed from storage,begins to decline if the plants have been stored for more than 1 week.Under hypobaric conditions relatively small but significant benefit wasrealized at pressures ranging from 100 to 150 mm Hg., but at 40 to 80 mmHg. the plants were kept for 4 weeks without any diminution in thesubsequent shelf-life of the blooms. Even tightly closed flower budssubsequently opened, whereas they aborted on plants removed from normalcold storage after about one week. Potted Easter lillies having flowerbuds just cracked open were stored at 2°C and pressures ranging from 40to 60 mm Hg., and remained in good condition for 6 weeks, whereas incold storage the leaves became chlorotic in 2 weeks. After hypobaricstorage the flowers had a normal shelf-life.

EXAMPLE 5

Non-rooted cuttings of chrysanthemums, consisting of nearly 100varieties, were stored at 40 to 150 mm Hg. and at 0°C to 4°C. Undernormal cold storage conditions such cuttings lose their viability within10 days to 6 weeks, depending upon the variety, even when they arewrapped in perforated polyethylene sheets. Under hypobaric conditions,storage was improved at 100 to 150 mm Hg., but at 40 to 80 mm Hg., thecuttings remained viable for more than 6 to 12 weeks without plasticwrap. Similar results were obtained with rooted cuttings, except thatunder normal cold storage conditions, they develop apical and leafyellowing within about 1 week, whereas this does not occur for about 12weeks at 40 to 60 mm Hg. Cuttings of geraniums were stored at 2° to 7°Cand pressures of 40 to 200 mm Hg. The optimal condition for storage was2°C at a pressure of 40 mm Hg. Under these conditions the cuttings werepreserved for 3 to 4 weeks and subsequently rooted without leafyellowing and abscission. At 2°C in cold storage the cuttingsdeteriorate within 2 to 7 days and when rooted lost their leaves andyellowed. Improved results are obtained with all cuttings if they areprotected by polyethylene wraps.

EXAMPLE 6

At 11°C under normal cold storage conditions, cut floral spikes of RedGinger wrapped in polyethylene, become unsaleable within 5 to 7 daysmainly because of deterioration of the leaves. Soon thereafter theflower also browns and desiccates. Under hypobaric conditions, asignificant benefit is obtained at a pressure higher than 100 mm Hg. atsimilar temperature even without plastic wrap. However, at 50 to 60 mmHg. both the leaves and bloom are preserved for 4 to 5 weeks.Subsequently, upon removal from storage, the floral spikes displayed anearly normal shelf-life.

EXAMPLE 7

The cut bloom of Heliconia latispathea developed necrotic spots andfaded within about 10 days when stored at 10°C under normal cold storageconditions. Hypobaric storage in the pressure range between 100 and 150mm Hg. prolongs storage life at this temperature, but at lowerpressures, such as 60 mm Hg., the bloom is preserved for as long as 40days.

EXAMPLE 8

Vanda joacquim blooms stored at 10°C under normal cold storageconditions faded and dehisced in about 2 weeks. Under hypobaricconditions in the pressure range between 100 and 150 mm Hg., and at thesame temperature, the orchid blossoms had enhanced storage life.However, the effect was not nearly as marked as that at lower pressures,such as 40 mm Hg. at the same temperature. Under these conditions, theblooms were preserved for more than 40 days and subsequently displayednearly normal vase-life.

EXAMPLE 9

Choquette avocados ripen in 8 to 9 days when stored under normal coldstorage conditions at 12°C. Hypobaric pressures ranging from 100 to 150mm Hg. at 12°C significantly extend storage life, but lower pressures inthe range between 40 and 80 mm Hg. are even more efficacious, allowingthe fruit to be stored for about 25 to 26 days. Similar results wereobtained at 15°C except all fruit ripened more rapidly than at 12°C. At10°C chilling damage soon became apparent, but those fruits underhypobaric conditions were the last to develop this disorder.

EXAMPLE 10

At 10°C under normal cold storage conditions Waldin avocados ripened in12 to 16 days. Hypobaric pressures ranging from 100 to 150 mm Hg. at10°C extended the storage life but were not nearly as effective anddesirable as pressures ranging from 60 to 80 mm Hg. at the sametemperature, which allowed the fruit to be stored for about 30 days. At12°C all fruits ripened more rapidly than at 10°C but those underhypobaric conditions were still preserved for the longest time. At 8°Cthe Waldin avocados experienced chilling damage, albeit those underhypobaric conditions were the last to develop this disorder.

EXAMPLE 11

At 8°C under normal cold storage conditions, Lula avocados ripened in 23to 30 days, whereas under hypobaric conditions in the pressure rangebetween 40 to 80 mm Hg. and at the same temperature, they were preservedfor 75 to 100 days. Higher pressures, in the range between 100 and 150mm Hg. at 8°C are not as desirable or effective in preventing ripeningof Lula fruit. Chilling damage occurred when the temperature was loweredto 6°C, but fruits kept under hypobaric conditions were the last todevelop this disorder. Booth 8 avocados stored at 8°C under normal coldstorage conditions ripened in about 8 to 12 days, whereas hypobaricpressures in the range between 40 and 80 mm Hg. at the same temperaturepreserved them for about 45 days. Between 10°C and 15°C ripeningoccurred progressively more rapidly but fruits stored under hypobaricconditions were still preserved for a much longer time than those keptunder atmospheric conditions. Chilling damage developed at 6°C, but onlyvery slowly in fruits stored at a low pressure. In general the storageof avocados is further improved and desiccation prevented if the fruitsare kept in plastic bags with small perforations.

EXAMPLE 12

Fresh green onions are difficult to store using only standardrefrigeration. At 0° to 3°C they remain in a saleable condition for only2 to 3 days. Under hypobaric conditions small but significant advantageis gained at pressures ranging from 100 to 150 mm Hg. at 0° to 3°C, butat 60 to 80 mm Hg the scallions remain in a saleable state for more than3 weeks.

EXAMPLE 13

The storage of green peppers, cucumbers, pole-beans and snap-beans wasbetter at 8° to 13°C under hypobaric conditions in the pressure rangebetween 100 and 150 mm Hg. than it was under normal cold storage atthose temperatures. However, at pressures ranging from 60 to 80 mm Hg.and at the same temperatures, better results were obtained. For example,at 8° to 13°C under cold storage, green peppers were preserved for 16 to18 days, whereas at 80 mm Hg. and 8° to 13°C. they remained fresh for 46days. At 8°C the peppers suffered cold damage both in cold storage andat 80 mm Hg. but at 80 mm Hg. the damage did not become apparent untilseveral days after they were transferred from the 80 mm Hg. to air. At5° to 8°C snap-beans spoil in 7 to 10 days using conventional coldstorage, but are preserved for about 26 days at 60 mm Hg. at the sametemperatures. In cold storage at 10°C, cucumbers can be kept for 10 to14 days, whereas at 80 mm Hg. and 10°C they are preserved for 49 days.Pole-beans can be preserved for 10 to 13 days at 8°C in cold storage,but remain in good condition for 30 days at 60 mm Hg. and 8°C.

EXAMPLE 14

Ripe strawberries, vars. Tioga and Florida 90, normally spoil within 5to 7 days if stored at 0° to 2°C by conventional cold storage means,whereas they are preserved for about 4 to 5 weeks at 0° to 2°C atpressures ranging from 80 to 200 mm Hg. Blueberries were kept up to 4weeks in a saleable condition under normal cold storage at 0° to 1°C,but remained in good condition for at least 6 weeks at that temperatureand pressures ranging from 80 to 200 mm Hg.

EXAMPLE 15

Iceburg lettuce remains saleable for about 2 weeks under cold storageconditions at 0° to 4°C, but remained fresh for about 4 weeks at thosetemperatures in the pressure range between 80 to 200 mm Hg. Not only didthe leaves stay crisper under hypobaric conditions, but also the buttsremained whiter. Pressures in the range between 150 to 200 mm Hg. arepreferred, because my present observation is that lower pressures caninduce a disorder known as "pink-rib".

EXAMPLE 16

Ruby red grapefruit developed peel pitting and lost its flavor within 4to 6 weeks when stored at 6°C by means of cold storage. Flavor wasretained but the peel still pitted during 90 days' storage at the sametemperature, using hypobaric pressures in the range between 80-150 mmHg. However at 250-400 mm Hg. peel pitting was prevented and flavorretention improved during 90 days' storage.

EXAMPLE 17

Mature green tomatoes were stored at temperatures of 7°, 10°, 13°, and16°C and pressures of 60, 80, 100, and 125 mm Hg. The optimal conditionfor storage was 13°C at a pressure of 80 mm Hg. Lower temperaturescaused chilling damage and higher temperatures hastened ripening duringstorage and increased the incidence of decay. Lower pressures causedinternal tissue damage and induced decay, whereas higher pressureshastened ripening significantly. The fruit was held for 8 weeks in agreen state at 13°C and about 80 mm Hg. under my method and subsequentlyripened normally at room temperature and atmospheric pressure undernormal shelf conditions. The same fruit kept at 13°C in cold storageripened within 2 weeks. It was found that washing the fruit inchlorinated water lowered the incidence of decay during my hypobaricstorage and subsequent ripening.

EXAMPLE 18

Freshly slaughtered ribs and rounds of beef were stored at 2°C andpressures ranging from 10 to 75 mm Hg. In each treatment samples wereeither stored naked or wrapped with a thin polyethylene,polyvinylchloride (PVC), or polyvinylidene chloride film. Meat stored incold storage in the plastic wraps developed slime, off-odor, a deepblack-red color, and discolored fat within 2 to 3 weeks. Weight loss wasabout 1%. Meat stored naked or wrapped at pressures ranging from 30 to75 mm Hg. did not store as well as wrapped meat in cold storage, and thelower the pressure the more rapidly the meat browned. However, meat thatwas wrapped with a plastic wrap and stored at 10 and 15 mm Hg. was wellpreserved for 45 days. At these pressures and at 2°C, the meat retainedits red bloom, failed to develop slime and off-odor, and maintainedwhite fat. Without the plastic wrap, severe desiccation occurred at allpressures tested with weight loss increasing progressively from 2% at 75mm Hg. to 6% at 10 mm Hg. When the meat was wrapped in plastic it lostonly 1% of its weight regardless of the pressure. Even better resultswere obtained when the meat was stored at minus 1°C and 8 mm Hg., but atminus 2°C there was some loss of bloom and evidence of freezer burn. Ipresently believe that lamb and veal will store with my method in thesame manner and with the same advantages of beef.

EXAMPLE 19

Pork loins and butts were stored at minus 1°C at pressures of 8 to 15 mmHg. The pork was wrapped in PVC film. After 3 weeks, the pork was stillin excellent condition. It had no odor, retained its initial color,showed no signs of slime formation or desiccation, and had a very lowpercent shrink. Under cold storage conditions, the pork spoiled in about7 days.

EXAMPLE 20

Fryer chickens, under normal cold storage conditions at minus 1° to 2°Cdevelop an unpleasant odor, experienced extensive shrinkage, and becamecovered with plaques of Pseudomonad bacteria within 3 to 7 days.Chickens stored at 50 to 150 mm Hg. benefitted from the treatment, butat 10 to 25 mm Hg. during a three-week period, odor, shrinkage andPseudomonas development were almost completely prevented. Use of a waterretentive wrap was beneficial, especially at the lower pressures.

EXAMPLE 21

Freshly harvested shrimp were wrapped in PVC film and stored at minus 1°to plus 2°C using pressures ranging from 8 to 125 mm Hg. Under coldstorage conditions, the shrimp became blotchy and developed a severbilgy and fishy odor within 4 to 6 days, whereas under hypobaraicconditions, it was well preserved for more than 15 days. The optimalpressure for storage was 8 to 25 mm Hg. although even 125 mm Hg. causedsubstantial benefit. At 8 mm Hg. pressure and the above temperatures,the shrimp did not become blotchy and had no bilgy or fishy odor after15 days.

EXAMPLE 22

Freshly caught mangrove snappers and grunts were stored at minus 1° toplus 2°C at pressures ranging from 8 to 150 mm Hg. The mangrove snapperswere gutted before storage whereas the grunts were stored intact. Ineach case, the fish were stored either naked, or wrapped in plastic. Incold storage the fish softened and developed slime and a foul odorwithin 4 to 6 days, whereas under hypobaric conditions it was preservedfor 2 to 3 weeks. The optimal storage pressure was 8 to 25 mm Hg.Plastic wrap (polyethylene or PVC) was required.

EXAMPLE 23

Unrooted cuttings of chrysanthemum, var. Blue Marble, were stored at2°C, 60 mm Hg. presssure and loosely wrapped with polyethylene. Each daythe pressure was cycled by being raised to atmospheric pressure forperiods of 0, 2, 4, 6, 8, 10, 12, or 14 hours, and then returned to 60mm Hg. During the period when the pressure was raised the air flow wascontinued and humidity kept high at a temperature of 2°C. In coldstorage after 10 days the cuttings became chlorotic and did not rootproperly. After 6 weeks storage, cuttings cycled for 14 hours each dayshowed slight leaf yellowing, but those cycled for 2 to 8 hours each dayappeared to remain greener than cuttings kept under continuous hypobaricpressure. Similar results were obtained when cuttings were cycled tocold storage conditions without humidification and air through-flow.

EXAMPLE 24

Unrooted cuttings of geranium were stored at 40 mm Hg., 2°C and looselywrapped with polyethylene. The vacuum was interrupted for 8 hours eachday. Under these conditions, the cycled cuttings were as well preservedduring a 3 week period as those kept continuously at 40 mm Hg., and bothtypes rooted properly without leaf yellowing. Cuttings kept in coldstorage at a temperature of 2°C yellowed after 2 to 7 days' storage andlost their leaves when an attempt was made to root them.

EXAMPLE 25

Cut carnations were stored at 1°C, at pressures ranging from 25 to 40 mmHg., loosely wrapped with polyethylene. Some of the blooms at 40 mm Hg.were cycled to atmospheric pressure for 4 hours each day. After 4 weeks,the blooms were removed and vase-life determined. A large percentage ofblooms stored in cold storage at atmospheric pressure and wrapped withpolyethylene failed to open properly, and those that did, had arelatively short vase-life. The vase-life of blooms kept continuously at25 or 30 mm Hg. was slightly better than that of blooms kept at 40 mmHg. Cycling the blooms to atmospheric pressure for 4 hours each day at40 mm Hg. diminished the incidence of tip burn.

EXAMPLE 26

Mature green tomatoes were stored at a pressure of 80 mm Hg. andtemperatures of 13°C. Control fruit held in cold storage ripened inabout 2 weeks. The vacuum in my method was interrupted daily for either0, 2, 4, 6, or 8 hours. Interrupting the vacuum for 8 hours had littleeffect on the storage life of the fruit during an 8 week period, but a2, 4, or 6 hour cycle slowed the rate of ripening and reduced theincidence of decay. Best results were obtained with a 4 hour cycle.

EXAMPLE 27

Potted azalea plants having dormant flower buds were stored at 4°C and40 mm Hg. The hypobaric pressure was interrupted for 8 hours each day.After 6 weeks, the plants retained their initial green color in spite ofthe fact that they were continuously in darkness. When placed in thelight under atmospheric conditions, they flowered in an additional 6weeks. Plants stored at 4°C under cold storage conditions developedsevere chlorosis even though they were watered as required andirradiated with artificial light.

EXAMPLE 28

Fore and hind quarters of beef were wrapped in PVC film for storage in ahypobaric trailer at a pressure of 8 to 10 mm Hg. The interior wallsurfaces and mechanism of the trailer were precooled to a temperature ofminus 1°C and thereafter kept at that temperature. The beef initiallyhad a temperature of 10°C at the time of loading it into the trailer.The temperature of the beef was dropped to minus 1°C within 18 hours,and weight loss measured 3 weeks later was only 2%, indicating that theinitial temperature reduction occurred without evaporative cooling ofthe beef. The condition of the beef was excellent even after 45-50 daysof storage. When a similar experiment was run at 16 to 18 mm Hg.,cool-down to minus 1°C required nearly 48 hours.

Some of the foregoing data as well as additional storage data arepresented by the following Tables A through F to illustrate thesuperiority of preserving MAM by my invention as compared with the priorart including my prior U.S. Pat. No. 3,333,967 and conventional,so-called normal cold storage at atmospheric pressure. In the tables,data obtained by my improved method is called "LPS". Where data, such astemperature or pressure is stated in a single FIGURE, it should be takenas a preferred value or an optimum mean.

                                      TABLE A                                     __________________________________________________________________________    NON-RIPE FULLY MATURE FRUIT                                                               Temp.   Storage Life -- Days                                                                          LPS Pressure                              Variety     (°C)                                                                           Cold Storage                                                                          LPS Storage                                                                           mm Hg.                                    __________________________________________________________________________    Tomato (mature green)                                                                     13      10-14   56*     80                                        Avocado, Choquette                                                                        13      8-9     25-26   40-80                                     Avocado, Waldin                                                                           10      12-16   22-30   60-80                                     Avocado, Lula                                                                             7       23-30    75-100 40-60                                     Avocado, Booth 8                                                                          7        8-12   45-60   40-80                                     Lime, Tahiti                                                                              7 to 10 14-35   60-90   80-150                                    Pineapple   7 to 10 10-14    28-30* 80-150                                    Apple, McIntosh                                                                           -1 to 0.6**                                                                            60-120 180-200 60                                        Apple, Red                                                                     Delicious  -1 to 0.6                                                                              60-120 240-270 60                                        Apple, Golden                                                                  Delicious  -1 to 0.6                                                                              90-120 240-270 60                                        Apple, Jonathan                                                                           -1 to 0.6                                                                             60-90   240-270 60                                        Pear, Bartlett                                                                            -1 to 0.6                                                                             75-90   150-180 50                                        Pear, Clapp -1 to 0.6                                                                             45-60   120-150 50                                        Peach, Cardinal                                                                           0 to 1  14-21   28-35   80                                        Nectarine   0 to 1  11-20   28-35   80-120                                    __________________________________________________________________________      *Storage life is limited in these instances by mold development at the       indicated times.                                                              **Throughout this specification figures for temperatures given without        plus or minus designations are intended to be read as above 0°C.  

                                      Table B                                     __________________________________________________________________________    RIPE, FULLY MATURE FRUITS                                                                 Temp.                                                                              Storage Life -- Days                                                                          LPS Pressure                                 Variety     (°C)                                                                        Cold Storage                                                                          LPS Storage                                                                           mm. Hg.                                      __________________________________________________________________________    Orange, Valencia                                                                          4    72      157      70-110                                      Grapefruit, Ruby Red                                                                      5 to 6                                                                             30-40    90-120*                                                                              250-400                                      Strawberry, Fla.                                                                          0 to 2                                                                             5-7      28-35*  80-200                                       90 and Tioga                                                                 Cherry, Sweet                                                                             0 to 2                                                                             14      28       80-200                                      Tomato (vine-                                                                             0 to 2                                                                              8-10   30-45   100                                           ripe)                                                                        Blueberry   0.5  28      42*      80-200                                      __________________________________________________________________________     *Storage life is limited in these instances by mold development at the        indicated times.                                                         

                                      Table C                                     __________________________________________________________________________    VEGETABLES                                                                              Temp. Storage Life -- Days                                                                          LPS Pressure                                  Variety   (°C)                                                                         Cold Storage                                                                          LPS Storage                                                                           mm Hg.                                        __________________________________________________________________________    Green pepper                                                                             8 to 13                                                                            16-18   46*     80                                            Cucumber  10    10-14   49*     80                                            Bean, Pole                                                                               7    10-13   30*     60                                            Bean, snap                                                                              5 to 8                                                                               7-10   26*     60                                            Onion, green                                                                            0 to 2                                                                              2-3     15      50                                            Lettuce, iceburg                                                                        0 to 2                                                                               14     28       80-200                                       __________________________________________________________________________     *Storage life is limited in these instances by mold development at the        indicated times.                                                         

                                      Table D                                     __________________________________________________________________________    FLOWERS -- CUT                                                                          Temp.                                                                              Storage Life -- Days                                                                          LPS Pressure                                   Variety   (°C)                                                                        Cold Storage                                                                          LPS Storage                                                                           mm Hg.                                         __________________________________________________________________________    Red ginger                                                                              11   5-7     28-35   50                                             Snapdragon                                                                               2    14     42      40                                             Heliconia latis-                                                               pathea   12    10     41*     60                                             Vanda joacquim                                                                          12    16     41      40                                             Carnation 0 to 2                                                                              10      91-98* 25-60                                          Rose       0    7-14   36      40                                             Chrysanthemum                                                                           0 to 2                                                                             6-8     21-28   70                                             Gladiolus  2   7       28      40-60                                          __________________________________________________________________________     *Storage life is limited in these instances by mold development at the        indicated times.                                                         

                                      Table E                                     __________________________________________________________________________    VEGETATIVE MATERIALS AND FLORAL CROPS                                                      Temp.                                                                              Storage Life -- Days                                                                          LPS Pressure                                Variety      (°C)                                                                        Cold Storage                                                                          LPS Storage                                                                           mm Hg.                                      __________________________________________________________________________    Potted chrysanthe-                                                              mums       0 to 2                                                                             7       28      60-80                                       Potted Easter lillies                                                                       2   14      42      40-60                                       Chrysanthemum cuttings                                                         (non-rooted)                                                                              0 to 2                                                                             10-42   42-84   40-80                                       Chrysanthemum cuttings                                                         (rooted)    0 to 2                                                                             7       42-84   40-80                                       Carnation cuttings                                                             (non-rooted)                                                                              0 to 2                                                                             90      270     40-60                                       Geranium cuttings                                                              (non-rooted)                                                                              0 to 2                                                                             2-7     21-28    40                                         Potted Azaleas                                                                              4   14-28   42       40                                         __________________________________________________________________________

                  Table F                                                         ______________________________________                                        ANIMAL PRODUCTS                                                               Temp.       Storage Life -- Days                                                                            LPS Pressure                                    Variety                                                                              (°C)                                                                            Cold Storage                                                                             LPS Storage                                                                            mm Hg.                                    ______________________________________                                        Beef   -1 to 2  14          50      8-15                                      Chicken                                                                              -1 to 2  7           21      8-50                                      Pork   -1 to 2  7           28      8-15                                      Shrimp -1 to 2  4-6        15-20    8-50                                      Fish   -1 to 2  4-6        15-20    8-15                                      ______________________________________                                    

While 100% relative humidity in the air that is caused to flow about theMAM is often best, at least in theory, it has been determined thatrelative humidities as low as about 80% are usefully permissible.Preferably, the humidity should be higher than about 90%. Even relativehumidities of about 80% are often difficult to maintain continuously anduniformly under hypobaric conditions in a large, commercial sizechamber. When the incoming air is cold it picks up relatively less watervapor in passing through a humidifier having a set water temperature,whereas when the incoming air is warm it picks up more water vapor andmay even drop condensate water on the produce and floor of the vacuumchamber upon cooling to the temperature of the chamber. Therefore somemeans of stabilizing the temperature of the incoming air is highlydesirable in order to avoid humidity fluctuations and commodity damagewithin the storage chamber. A problem also arises in connection with thefact that not all of the incoming air necessarily enters through thehumidifier. In some instances, as when incoming air is used to drivepneumatically actuated equipment in the chamber, it may bypass thehumidifier initially and intentionally. In other instances, for examplein a large commercial structure such as a concrete warehouse, a certainamount of in-leakage of air through the surfaces and seams isunavoidable, and this air upon expanding within the structure will havean extremely low content of water vapor. vapor tends to

For these reasons it is highly desirable to provide means forrecirculating air within the vacuum chamber, so that all or some part ofthe air repeatedly passes through the humidifier and therefore,regardless of its route of entry into the chamber, it will becomesaturated, or sufficiently saturated, with water vaopr at thetemperature within the chamber. Recirculating the air through thehumidifier has the additional advantage that it tendsto compensate forinefficiency in the humidification system. Generally I have found thateven when wicks, atomizers or other devices are used to increase theefficiency of the humidification process it is difficult, if notimpracticable, to saturate the incoming air in a single pass through ahumidifier of economically modest capacity without increasing the watertemperature to a high value.

A further factor influencing the attainment of a constant, high relativehumidity in a vacuum storage chamber is the aforementioned evaporativerefrigeration effect. In a relatively small apparatus the high surfaceto volume ratio of the water bath favors sufficient heat exchangebetween the water and surrounding atmosphere to cause the watertemperature to approach that within the storage chamber in which thewater bath is situated. Normally, not less than about 1/4 chambervolumes of rarified air should be passed through the hypobaric chambereach hour to flush away undesirable vapors, and gases, such as ethylene,carbor dioxide and off-odors produced by the stored MAM ormicroorganisms growing thereupon. Such a rate of through-flow of airwill, normally, also supply sufficient oxygen to replace that consumedby respiration. This through-flow or through-put of air does notsubstantially lower the water temperature if the chamber volume to watervolume is small, for example from about 20:1 to about 200:1, because ofthe low rate of cooling and rapid heat exchange between the water vesseland surrounding atmosphere. However, in a large apparatus where theratio of the chamber volume to water volume may be in the order of 1000or more to 1, the cooling effect is relatively much greater and the heatexchange between the water reservoir and surrounding atmosphere lessrapid because of an unfavorable surface to volume ratio in the largerwater reservoir. Under these and other adverse conditions, evaporativecooling tends to reduce the water temperature so that a humidity of even80% cannot be sustained in the vacuum storage chamber without doing morethan merely passing the input air through a body of water once.

The present invention continually attains a desired relative humiditypreferably by preconditioning the incoming air to have a temperatureclose to that in the storage chamber and maintaining the temperature ofthe water in the humidifier at least equal to or higher than that of theair in the storage chamber. It is also preferable to prolong or multiplythe contact time between the circulating air and the water, for exampleby recirculating the air through the humidifier or by utilizing sprayatomizers. I prefer that heat be supplied to the water to maintain itstemperature at or above that of the temperature in the storage chamber,for example, from about 2° to about 20°C higher. The heat energy may beobtained from different sources or a combination of sources.

For example, heat may be garnered from the interior of the chamber byvarious means such as circulating heat exchange fluid in tubes which arein intimate contact with an internal surface, preferably metal, of thechamber and/or recirculating the gas within the chamber and contactingit with a heat exchange surface to transfer the heat of the gas to thewater in the humidifier of my et al. U.S. Pat. No. 3,810,508. While thislatter method of heating the water is useful and serves the dualfunction of keeping the chamber cool, by itself it never quite raisesthe temperature of the water to closer than a few degrees lower thanthat of the chamber air. Under these conditions, supplementary heat mustbe added. This can be done in a second stage of humidification bysubsequently contacting the air with a second water source maintained 2°to 20°C hotter than the chamber air temperature. The air preferablypasses through the second humidification stage without experiencing apressure drop; otherwise a back pressure is created in the first stagehumidifier and its efficacy is greatly diminished. A convenient solutionto this problem is to spray humidify with warm water in the secondstage.

Alternatively as indicated in the accompanying drawing, the additionalheat may be supplied all in a single stage by spray humidifying andheating the water in the conduit leading from the water reservoir to thespray nozzle. When use is not made of the refrigeration effect and/orwhen heat rather than refrigeration is required to maintain the desiredchamber temperature, heat may be added directly to the water in thereservoir as by heating the water with an electric immersion heater.

Loss of water from the stored matter tends to occur, especially during aprolonged storage period, particularly at pressures below 100 mm Hg evenwhen the relative humidity of the gas in the chamber is at or close to100%. The rate of escape of water from the commodity is roughlyproportional to the rate of movement of air over the commodity, andtherefore under these conditions I prefer that air circulation orrecirculation be kept at or near the minimum required to maintaintemperature and humidity uniformity in the storage chamber, whilesupplying adequate oxygen and flushing away released gases. Water lossof this nature can be slowed or prevented by wrapping the stored matterin water-retention means such as in sheets of synthetic, resinousplastic or a perforated bag of the same material. Synthetic, resinousplastics which can be used include polyethylene, polypropylene,polyvinyl chloride, polyvinylidine chloride (Saran wrap), polyvinylbutyral, polymethacrylate esters, and the like as well as copolymersthereof. Polyethylene is preferred because it tends to restrict themovement of water while allowing the passage of gases such as oxygen,ethylene and carbon dioxide. Such plastics do not interfere with theoperation of the hypobaric process but to the contrary tend to prolongstorage life of certain commodities. For some commodities, beef forexample, at pressures below 100 mm Hg. wrapping or the equivalentpresently appears to be essential. With avocados and bananas, ripeningis actually slowed by plastic wraps when the commodity is placed in avacuum chamber maintained in accordance with the present invention.

A unique characteristic of my nw hypobaric system is that, because therarified air is maintained at or near its dew point, the water vaporpressure is kept at a constant value regardless of pressure. The amountof water vapor present in the rarified air is a function of temperature,not pressure; whereas the amount of air present is directly related topressure with the reservation that at pressures below about 100 mm Hg. asignificant correction for the amount of water vapor present must bemade.

The stored commodities can be cooled down most rapidly in the hypobaricenvironment by progressively reducing the hypobaric pressure as thetemperature of the commodity is reduced so that the pressure always isonly slightly higher than the vapor pressure of the water in thecommodity. At pressures close to the water vapor pressure in thecommodity the thermal conductivity of the hypobaric atmosphereapproaches that of pure water vapor, which is thirty times greater thanthat of air assuming no substantial Dewar effect occurs. The commodityshould not be exposed to pressures below its permissible minimal levelfor such a period of time as will cause internal damage by oxygendeficiency or other deleterious actions by low pressure. Rapid cool-downcan be accomplished by monitoring the pulp temperature of the commoditywith a temperature probe inserted therein and adjusting the pressureregulator valve to a pressure that is slightly higher than the vaporpressure of water at the pulp temperature.

Alternatively, for many commodities the cool-down process can be carriedout at the optimal hypobaric storage pressure for the particularcommodity. Various combinations of progressively decreasing pressure andfixed pressure cool-down can also be used. The use of a plastic waterretentive wrap improves the cool-down process by maintaining the airbetween the wrap and the commodity near its dew point, thus providinghigh thermal conductivity thermal regardless of the conductivity of thehumid hypobaric air outside the wrap. During cool-down the temperatureof the recirculating air always is intermediate between that of thecooling surface and that of the hot commodity, so the water vaporcontent of the recirculating air is much lower, and the absolute amountof oxygen and nitrogen somewhat higher than that in the warm atmospherebetween the commodity and plastic wrap.

If the hypobaric pressure is close to the vapor pressure of water in thecommodity, the gas between the commodity and the wrap is composed almostentirely of water vapor, whereas the recirculating air has a relativelylow water content and thermal conductivity. Under these conditions heatwill flow from the commodity very rapidly. Since the speed of cool-downis limited by the rate at which heat leaves the commodity, provided theheat can be transferred efficiently to a cooling surface, themaintenance of a high humidity in close proximity to the commodityfacilitates an extraordinarily rapid cool-down under hypobaricconditions. This cannot be accomplished at atmospheric pressure becausewater vapor does not substantially increase the thermal conductivity ofair when the pressure is greater than 100 mm Hg.

As the pressure is reduced below 100 mm Hg. the proportion of watervapor molecules relative to oxygen and nitrogen molecules increasesalmost logarithimically until at a pressure equal to the vapor pressureof water at the operational temperature, all the molecules are watervapor. The thermal conductivity of a gas is independent of pressureuntil the Dewar effect sets in at some low pressure, usually between 0and 25 mm Hg., the exact value being a function of the distance betweenthe cooling surface and the body being cooled. The thermal conductivityof gas mixtures such as water and air cannot be calculated additivelyfrom the corresponding values for the pure components since it dependsupon the mean free path, and this magnitude for one species of moleculesis influenced by the presence of another species. It can be calculatedfrom the Enskog equation for viscosity of gas mixtures since thermalconductivity is directly related to viscosity.

The thermal conductivity of a dry gas mixture is substantiallyindependent of temperature at any pressure. Thermal conductivity of anair-water mixture is relatively constant from 760 mm Hg. to about 100 mmHg., below which its thermal conductivity rises in a nearly logarithmicmanner because of the fact that water vapor conducts heat about 30 timesfaster than nitrogen or oxygen molecules. Thermal conductivity of watersaturated air is markedly dependent on temperature at pressures below100 mm Hg., because temperature than influences the percent of water inthe mixture. Consequently when the absolute pressure equals the vaporpressure of water, the thermal conductivity of water saturated air is 30times greater than dry air at any pressure, or water saturated air atpressures greater than 100 mm Hg.

Experimentally I have determined that in a commercially sized hypobarictrailer the Dewar effect does not set in at 8 mm Hg. absolute pressure.If the Dewar effect became pronounced, the thermal conductivity woulddecline to a very low value. As the pressure of dry air is reduced, theheat capacity declines proportionately, and accordingly at pressureslower than 100 mm Hg. it becomes impossible to refrigerate theatmosphere by conduction without resort to an impractical rate of airrecirculation or an excessively large cooling surface area. The heatcapacity of water vapor (0.5 cal/gm/°C) is about twice that of air (0.24cal/gm°C) so water-saturating the air only slightly alleviates theproblem of reduced heat capacity at low hypobaric pressures. However,the most important factor making conductive heat exchange possible atlow hypobaric pressures is the increased thermal conductivity below 100mm Hg. caused by water vapor, for the rate of heat transfer isproportional approximately to thermal conductivity times heat capacity.I have discovered that it is this remarkable physical property ofwater-saturated air at low pressures which enables me to use myhypobaric process at pressures lower than 100 mm Hg. As mentioned abovethe cool-down of hanging quarters of PVC wrapped beef from 10° to minus1°C took only 18 hours in the pressure range of 8 to 10 mm Hg. Whereasit took 48 hours in the pressure range of 16 to 18 mm. Hg.

During and after cool-down the transfer of heat to the refrigerationmeans can be improved by recirculating air within the hypobaric chamber,but often the through-put of air provides sufficient circulation byitself. The requirement for recirculation varies with the pressure andstorage temperature, and is preferably determined empirically. A coolingadvantage also is gained by increasing the surface area of the coiledfins or plates of the refrigeration surface. In the case of a trailer orcargo container, this can be accomplished by making the entire innerwall a cooling surface, using heat exchange fluid such as glycol orbrine to cool it by passage through conduits embedded in or in closecontact with the walls. It is important that the temperature of therefrigerant entering and leaving the storage compartment be as nearlyequal as possible. If there is a large difference between the in-and-outtemperature the humidity in the chamber will be reduced because waterwill condense on the colder incoming refrigerant line. The dew-point inthe chamber can be no higher than the temperature of the incomingrefrigerant. When using cooling tubes carrying liquid refrigerant, incontact with the metal walls of a hypobaric container, I have found itadvantageous always to pair the tubes so that one member carriesincoming coolant, and the other coolant which is leaving. In this mannera more uniform temperature is maintained in the vicinity of the coolantlines, and the humidity kept at the highest desirable value.

In the accompanying drawings, FIG. 1 illustrates presently preferredapparatus for carrying out my hypobaric storage methods andimprovements. An insulated vacuum chamber 10 is provided which iscapable of withstanding external pressure safely in excess of anypossible maximum atmospheric pressure. A humidifier tank 11 containingwater 12 is preferably or conveniently situated within vacuum chamber10. Air is continuously, or intermittently evacuated from the chamber bya conventional vacuum pump P at a rate influenced in part, if desired,by the degree of closure of valve 14 in line 15. The rate of flow of airthrough the chamber and the pressure in the chamber may be adjustedprimarily by valves 14 and 30 and regulator 32 to provide between about0.25 to 10 changes of chamber air per hour. Atmospheric air enters thesystem through conduit 16, on the right as viewed, passing through anair filter 17. The filter may contain charcoal, inert pellets coatedwith permanganate salt (marketed under the trademark "Purafil"),molecular sieves or other purifying agents alone or in combination toremove atmospheric contaminants such as carbon monoxide and ethylene.

These gases and various other unsaturated atmospheric contaminantsinfluence biological processes in such a way that they cause fruitripening, scald, senescence, aging, abscission or twisting of leaves,stems and floral parts, fading of flowers, chlorosis of leaves, andcertain physiological disorders such as sepal wilt in orchids, "sleep"of carnations, and browning of lettuce. Fortunately, when atmosphericair enters the vacuum chamber it expands so that the partial pressure ofany contaminant is reduced proportionately. Under hypobaric conditions,if the pressure in the vacuum chamber is sufficiently low, theconcentrations of contaminants tend to be reduced to values lower thanthose needed to influence biological processes adversely except underconditions of severe atmospheric pollution. In that event, filter 17will be called upon for extra duty or increased capacity. The rate offlow of incoming air at atmospheric pressure is preferably indicated bya rotameter 18 or other flow meter.

Preferably the incoming air is preconditioned to or toward thetemperature within the vacuum chamber 10, by passing it through a heatexchanger 20 where it exchanges heat with the "used" air leaving thevacuum chamber. Preconditioning in heat exchanger 20 may lower thetemperature of the incoming air if it be warmer, without increasing theoverall refrigeration requirement for the chamber. To furtherprecondition the input air, it is passed through section 21 of conduit16 which may be longer than, or comprise a plurality of, the singlelength as shown, and has intimate contact with the inner wall suface ofthe chamber 10.

The input air flows through parts 22, 23, 38 and 39 extending fromconduit 16 to enter the chamber 10. All parts of the conduit 16downstream of heat exchanger 20 which lie outside of the chamber shouldbe insulated as suggested at 19. Branch conduit 22 leads to the sideinlet of the annular high pressure injector jet of a conventionalventuri-type air-mover 24. A relatively small volume of air atatmospheric pressure flows through branch part 22 and through theinjector of the air-mover at high velocity because it drops in pressurefrom atmospheric to the low pressure obtaining within the vacuum chamberand within humidifier 11. This induces a large flow of air through thelow pressure body of the air-mover 24. Air is thus drawn into inletelbow 28 from within the chamber 10 and into humidifier 11 above thewater level of reservoir 12 whence it flows upwardly through water sprayfrom nozzle 35 and out from the humidifier at outlet 25. The humid airis recirculated in this way throughout the chamber 10 as indicated byarrows 27. A fraction of the moving air is withdrawn from the chamber 10by vacuum pump P through conduit 15, and the remainder is preferablydrawn to the air-mover via elbow 28 for continued recirculation. Therecirculating air preferably passes over or through a heat exchange coilC of a conventional refrigeration and heating system not shown, whichprovides heating and cooling, as need be, responsive to a conventionaltemperature sensing device, not shown, located within chamber 10. Aircan also be moved by conventional fans or blowers disposed in thechamber.

The rate of recirculation of chamber atmosphere is controlled by valve30 in branch conduit 22, and can be judged from reading vacuum gauge 31.Incidentally, valve 57 is always closed except when chamber pressure isto be raised to atmospheric as described below.

The direction of air movement need not be the same as that shown in thediagram wherein air is expelled over the top of the load L.Alternatively, as shown in FIG. 2, and more fully described below, thedirection of air flow may be altered by locating the air-mover 24 up at25 to direct recirculated air into the top of the humidifier so that airleaving the humidifier may be directed under the floor or a false floor,of the vacuum chamber to force air below the load and then verticallyupwardly through the load.

Referring back to FIG. 1, incoming air also may be directed to passthrough branch conduit 23 to and through a vacuum regulator 32, andthence preferably through check valve 26, to humidifier 11, bubblingthrough the water reservoir 12, if desired, before comingling with therecirculating air in the upper part of the humidifier. Regulator 32,which may be conventional diaphragm type, continuously senses thepressure within the vacuum chamber through a line 29 and compares thisto atmospheric reference pressure, allowing just enough air as shown byflowmeter 42 to enter humidifier 11 to maintain a set difference betweenthe chamber and atmosphere. To maintain pressure within the vacuumchamber at an absolute rather than relative value, it is necessary toestablish an absolute reference pressure in the regulator. This can beaccomplished by having a sealed, absolute vacuum in a vessel, not shown,as the reference pressure, or by using such an absolute pressureregulator, not shown, to establish an absolute pressure in the referenceside of regulator 32 which is lower than any anticipated atmosphericpressure but higher than the desired pressure intended to be maintainedin chamber 10 as indicated by absolute pressure gauge 33. Absolutepressure regulation is not highly essential in stationary facilities,but is recommended in transportable containers which are likely toencounter severe and frequent fluctuations in atmospheric pressure.

Air entering humidifier 11 through conduit 23 is humidified as itbubbles through the water 12 and/or is exposed to water spray fromconventional siphon-fed pneumatic spray atomization nozzle 35. Thenozzle is actuated by the pneumatic force and motion of atmospheric airas it expands, flows and decreases in pressure to lift water to thenozzle in conduit 36 and eject the water into the air and water vaporspace 13 of the humidifier above the water 12. In-line water filter 37prevents clogging of the nozzle. Air is supplied to the nozzle throughparts 38 and 39 of conduit 16. The rate of air and water utilization bythe nozzle depends upon the pneumatic force applied, which can beadjusted by valve 40 to a desired pressure, as read on a vacuum gauge41. Relatively small amounts of air are consumed in a spray nozzle sonumerous nozzles can be used if required. Alternatively, hydraulicatomizing nozzles may be used, in which case, a water pump, not shown,would be required to circulate water to the nozzles from the reservoir12 in the humidifier 11.

As discussed above, evaporative cooling causes the water temperature inhumidifier 11 to tend to be lower than the air temperature in chamber10, thereby lowering the humidity in the chamber. My preferred solutionto this problem is to heat the the water. In the preferred embodimentshown in the drawing, the water 12 is heated by an electric immersionheater 47 responsive to a thermostat 48 and water temperature sensingelement 49, so that the water temperature can be held at any desiredvalue. To attain desired relative humidity in the chamber 10, I preferthat the water be kept at or about 2° to 20°C warmer than thetemperature of the air in the chamber. The exact temperature for optimumhumidity depends to considerable extent upon the amount of airrecirculation and re-humidification through the humidifier, and theefficiency of the humidification process.

When valve 30 is closed so that no air recirculation occurs, all airenters the chamber through conduits 23, 38 and 39. If valve 40 is alsoclosed, so that the spray nozzle 35 cannot function, the efficiency ofthe humidification process will be reduced to the single pass of airbubbling through the water 12. Consequently the temperature of water 12will have to be raised relatively high above chamber air temperature toprovide saturated or satisfactory humidity in the chamber. If the spraynozzle 35 is also operated by opening valve 40, the water temperaturecan be adusted to a lower value, and if the air-mover 24 is alsooperated by opening valve 30, a still lower water temperature willsuffice to achieve saturated or satisfactory humidity in the chamber.

An additional heater 61 located in conduit 36 is made responsive totemperature sensing element 62 by the thermoregulator 63. This heaterserves to warm the water entering the spray nozzle 35 to a desiredtemperature. When evaporative cooling is sought or employed to cool, orhelp cool, the vacuum chamber 10, the water temperature in reservoir 11should be lower than the air temperature in the chamber. Then tomaintain desirable relative humidity the water ejected from the spraynozzle is heated to a higher temperature than the air in the chamber.

Similarly relative humidity can be controlled and held at values lowerthan 100 %, by setting the water temperature to the relatively coolvalue required to maintain the desired lower humidity. Control of thehumidity at a value slightly lower than 100 % has the advantage withsome commodities that it decreases mold development without causing anunacceptable amount of desiccation.

It is possible to operate the apparatus at air temperatures even lowerthan minus 2°C without danger of freezing the water if the watertemperature is raised more than 2°C by adding heat. A non-volatileantifreeze compound ca be mixed with the water to prevent freezingduring periods of inoperation, and a desired humidity still can beattained in spite of the additional lowering of the vapor pressure ofthe water in the humidifier caused by the dissolved solute. This isaccomplished by raising the water temperature to a higher value thanotherwise would be necessary in the absence of the antifreeze compound.

Humidification is improved by continuously recirculating at least partof the air in chamber 10 through the humidifier. Water evaporated fromthe humidifier is replaced from an external source through conduit 52 asrequired. In a preferred method of water replenishment, the entry ofwater through an inlet conduit 52 is made responsive to a standard floatleveling device 53 which senses or equals the water level in humidifier11. Air pressure equalizing bypass line 54 connects water level device53 with chamber 10. To avoid accumulation of salt, scale and otherimpurities in the water 12 of the humidifier due to continued waterevaporation, water entering at 52 should be purified, for example bypassage through a reverse osmosis membrane and/or deionizing resins ofthe mixed bed type. Impure water should be removed as may be necessaryfrom time to time, as by a positive displacement pump 65 via conduit 66.Removal is facilitated during intervals when the pressure in chamber 10is raised to atmospheric as taught above.

The humidifier need not be located within the vacuum chamber so long asits substantial function and results as herein described are preserved.If it is located externally to the vacuum chamber and appropriatelyconnected therewith, it and all associated parts and connections shouldbe insulated and constructed to withstand atmospheric pressure. Placingthe humidifier outside the vacuum chamber is convenient in that thispermits easy access at atmospheric pressure to the humidifying andair-moving equipment, for control thereof and attendance thereupon.When, however, atmospheric air is admitted to the chamber periodicallyattention to the equipment is greatly facilitated within the chamber.

To raise the pressure in chamber 10 to atmospheric and to open thechamber for access, valve 14 in line 15 and valve 56 in line 16 areshut, vacuum pump P is stopped, and valve 57 in bypass line 58 and valve59 in conduit 60 are opened. Valve 59 admits atmospheric air andpressure to the chamber 10. Valve 57 admits air at chamber pressure tothe line between check valve 26 and regulator 32. Conventional,automatic timing and control apparatus, not shown, may of course, beemployed to start and stop the pump and open and close the valves toeffect any desired and selected cycle and change in pressure in thecontainer.

FIG. 2 illustrates another preferred apparatus for carrying out myhypobaric storage methods and improvements in which there aredifferences with respect to the apparatus of FIG. 1 that will presentlyappear. Air-mover 71 corresponding to air-mover 24 is operated byincoming air in line 22 as in FIG. 1. However, air-mover 71 dischargesinto an upper portion of the humidifier 70 and induces air from theupper levels of chamber 10 to or toward the humidifier and at or acrossan adjustable damper or baffle 72 which is disposed to admit all or partof the air emitted by air-mover 71 to the upper space 13 of thehumidifier 70 or divert all or part of such air downwardly into the pipe73 to the lower part of the chamber 10 beneath the load L of storedmaterial 45 therein. Air leaves the humidifier 70 through a side port 74into a conventional filter-mist eliminator 75 which spins out entrainedwater droplets. The water collects at the bottom of the eliminator 75and drains by gravity back to the water supply 12 through conduit 76.Air leaves the filter-mist eliminator 75 through conduit 77, rejoiningin conduit 73 the air, if any, which bypassed the humidifier. A fractionof the moving air 78 is withdrawn from the chamber 10 by vacuum pump Pthrough outlet conduit 15, and the remainder is drawn to the air-mover71 via inlet 79 to continue recirculation. If all of the recirculatingair emitted by air-mover 71 were directed through humidifier 70 andshould the air-mover and humidifier be of sufficient capacity, the aircould cause an undesirable carry-over of entrapped water droplets fromthe humidifier. To control this, baffle 72 is provided to divert part ofthe recirculating air around the humidifier.

In FIG. 2, heat exchanger 20, line 15, and pipe 16 near the exchangermay be insulated as at 80. I have found that impure water may be drainedconveniently from the reservoir during periods of inoperation when thevacuum in chamber 10 has been released as by draining line 51 throughconduit 81 and valve 82. In the form of my apparatus in FIG. 2, I prefernot to use the check valve 26 in line 23 as shown in FIG. 1, but insteaddispose the regulator 32 in extended aspects of lines 23 and 29 to ahigher elevation than the water level in the humidifier 70 as shown atthe left of FIG. 2.

The moisture content of the air in chamber 10 is measured by device 85,which may be any type of conventional relative humidity indicatorinsensitive to atmospheric pressure. For example, a wet and dry bulbresistance or other type of thermometer, or a membrane actuatedhygrometer may be used. I prefer an electronic dew point indicatorhaving a heated, wire wound, salt coated bobbin 83 as a sensor, and athermister probe 84 to measure the air temperature in chamber 10. Ifdesired, the moisture content of the air in chamber 10 can becontinuously controlled by making heaters 61 and/or 47 responsive tomoisture sensing device 85, conveying signals from device 85 through,supplementing and/or superceding thermostats 63 and 48 respectively, vialine 86 upon closing switch 87 for that purpose.

Storage of MAM at my newly discovered low absolute pressures taughtherein facilitates diffusive escape of vapors such as carbon dioxide,ethylene, farnescene, ethanol, acetaldehyde, and various metabolic wasteproducts produced within plant material, as well as various putrefyingodors produced within or upon animal matter. Low absolute pressure ofair also decreases the partial pressure of oxygen available to supportmetabolic activity, and tends to prevent oxidation of myoglobin andhemoglobin to metmyoglobin and methemoglobin. The growth of aerobicbacteria and certain molds is retarded, and various undesirable animalforms, such as insects and nematodes, which sometimes infest and developin stored animal and plant commodities, may be destroyed both by lack ofoxygen and hypobaric pressure.

The number of chamber volumes of air passed per hour through the storagechamber is ordinarily not critical, except for the cost of pumping,provided it is sufficient to prevent accumulation of undesired vapors inthose cases when such vapors diffuse from the matter. Rapidrecirculation of air internally within the chamber even as often as 300times per hour improves heat exchange to and from the stored MAM. Duringusual operation through-put of air ranges preferably from about 1/4 to10 chamber volumes per hour regardless of the rate of internal airrecirculation.

While I have disclosed preferred forms of methods of and means forpracticing my inventions, other forms and embodiments may occur to thoseskilled in the art who come to know and understand my inventions, allwithout departing from the essence and substance thereof. Therefore I donot want my patent to be restricted merely to that which is specificallydisclosed herein, nor in any other manner inconsistent with the progressby which the art has been promoted by my improvement.

I claim:
 1. The method of preserving metabolically active mattercomprising placing said matter in air in an enclosed space, maintainingthe relative humidity of said air between about 80 and 100%, addingfresh air to and moving humid air from said space, controlling thetemperature of said matter, maintaining the pressure of said air betweenabout 4 mm. Hg. and 100 mm Hg. absolute, and correlating pressure,temperature, air movement and relative humidity in said space withrespect to the nature of the particular matter being preserved.
 2. Themethod of claim 1 wherein the air temperature is maintained betweenabout minus 1° and plus 13°C.
 3. The method of claim 1 wherein thestored matter comprises animal products such as beef, chicken, pork,shrimp and fish; vegetative materials and floral crops such as pottedchrysanthemums, potted azaleas and Easter Lilies, rooted and non-rootedchrysanthemum cuttings, rooted and non-rooted carnation and geraniumcuttings; cut flower such as snapdragon, carnation, roses,chrysanthemums and gladiolus; and green onions, apples, pears,nectarines and peaches, and the air temperature is maintained betweenabout minus 1° and plus 4°C.
 4. The method of claim 3 wherein the storedmatter comprises said animal products and the pressure is maintainedbetween about 8 mm and 50 mm Hg.
 5. The method of claim 1 wherein thestored matter comprises mature green tomatoes, avocados, green peppers,cucumbers, snap and pole beans, red ginger, helicania latispathea, andvanda joaquim, and the temperature is maintained above about 5°C andbelow about 13°C.
 6. The method of claim 1 with the steps of coolingsaid air and recirculating cool humid air over and about said matter. 7.The method of claim 1 with the step of wrapping said matter withmaterial tending to retain water vapor in the space between the surfaceof said matter and said material.
 8. The method of claim 1 with thesteps of maintaining said hypobaric conditions predominately and raisingthe air pressure to a value between about 100 mm Hg. and 760 mm Hg.periodically.
 9. The method of claim 1 with the step of humidifying saidair by adding water thereto at a temperature not substantially less thanthe temperature of the air in said space.
 10. The method of preservingmetabolically active matter comprising placing said matter in air in anenclosed space, maintaining hypobaric storage conditions of about 4 to400 mm Hg. and predetermined related conditions of temperature,hypobaric pressure, relative humidity and air flow correlated topreserve said matter in said space at certain times, and maintainingconventional cold storage conditions in said space at other times toalleviate undesirable adaptation of said matter to the low oxygenenvironment of said hypobaric storage conditions.
 11. The method ofclaim 10 wherein said conditions are modified cyclically.
 12. The methodof claim 10 wherein said cold storage conditions are maintained forabout 1 to 14 hours per day at least on certain days.
 13. The method ofcooling metabolically active matter rapidly in air composed of gases andwater vapor in an enclosed space at sub-atmospheric pressure comprisingthe steps of reducing the pressure of said air to between 4 and 100 mmHg., cooling said air, increasing the thermal conductivity of said airby increasing the relative humidity thereof to between 80 and 100%,simultaneously increasing the relative proportion of water vapor to saidgases in said air, and moving said cold, hypobaric, heat conductivemixture over said matter in thermal contact therewith.
 14. The method ofclaim 13 wherein said matter is cooled rapidly to a temperaturedifferent from its desired steady-state storage temperature with thestep of increasing the temperature and pressure employed for rapidcooling to the temperature, humidity and pressure desired forsteady-state storage.
 15. The method of claim 13 with the step oftemporarily interrupting the said conditions employed for cooling thematter rapidly by increasing the temperature and pressure thereof topreserve said matter from cold damage, freezer burn, or anaerobiosisduring the cool-down process.
 16. The method of claim 13 with the stepof wrapping said matter with material tending to retain water vapor inthe space between the surface of said matter and said material.
 17. Themethod of claim 13 wherein said matter has a water content with the stepof reducing the pressure in said space as the temperature of said matteris reduced.
 18. The method of preserving metabolically active matterother than mature, but less than fully ripe, fruit which producesethylene and is ripened thereby, in an enclosed space in movinghypobaric air at 4 to 400 mm Hg. comprising maintaining predeterminedrelated conditions of temperature, hypobaric pressure, relative humidityand air flow co-related to preserve said matter, adding fresh air to andwithdrawing humid air from said space, providing a supply of water,adding water vapor to said fresh air from said supply, and maintainingthe temperature of said supplied water substantially as warm as thetemperature of said air.
 19. The method of claim 18 in which thetemperature of water in said body of water is 20°between about 2° to 20Chigher than the temperature of the air in said space.
 20. The method ofclaim 18 in which humid air is taken from and fresh air is added to saidspace, with the step of preconditioning said air entering said space toa temperature, pressure and humidity proximate the correspondingconditions in the air in said space.
 21. The method of claim 18 in whichsaid heated water is introduced into said air through an hydraulic wateratomizer with the step of exposing the water supply to atmosphericpressure to force the water into said space.
 22. The improvement ofclaim 18 with the steps of removing low pressure air from, and supplyinghigher pressure fresh air to, said space, humidifying part of said airon its way to said space, and inducing circulation of humid air withinsaid space by the movement of another part of said air into said spacethrough a pneumatically actuated air mover.
 23. The method of claim 18with the step of wrapping said matter in water-retention means enclosingsaid matter and retarding water-loss therefrom and enhancing heattransfer therefrom.
 24. The method of claim 18 with the step of coolingsaid organic matter by progressively decreasing the pressure of said airas the temperature of said commodity is lowered.
 25. The method forpreserving metabolically active matter other than mature, but less thanfully ripe, fruit which produces ethylene and is ripened therebycomprising:a. placing said matter in air in an enclosed space, b.humidifying said air, c. moving said humid air in said space in contactwith said matter, d. maintaining hypobaric pressure in said space withinthe range of about 4 mm Hg. to about 400 mm Hg., maintaining the airtemperature in said space from about minus 2° to about 15°C, maintainingthe relative humidity in said air between about 80 and 100%, andexhausting air from said space at a rate between one-fourth to ten timesthe volume of said space at the reduced pressure thereof per hour, e.said hypobaric pressure, humidity, air flow and temperature beingdependent upon the nature of said matter and co-related to preserve saidmatter for a prolonged period of time, as compared with conventionalcold storage, and f. recirculating said air in said space.
 26. Themethod of claim 25 in which said matter comprises non-ripe fruits suchas limes, pineapples, peaches and nectarines, said pressure is fromabout 80 to about 150 mm Hg., and said temperature is within the rangeof about 0° to about 10°C.
 27. The method of claim 25 in which saidmatter comprises ripe fully mature fruit such as Valencia oranges, rubyred grapefruit, pineapples, strawberries, sweet cherries, vine ripetomatoes and blueberries, said pressure is from about 70 to about 400 mmHg., and said temperature is within the range of about 0° to about 14°C.28. The method of claim 25 in which said matter comprises iceburglettuce, said pressure is from about 80 to about 200 mm Hg., and saidtemperature is within the range of about 0° to about 2°C.
 29. The methodof claim 25 including providing water retentive means substantiallyenclosing said matter to retard water loss therefrom and increase heattransmission therefrom.
 30. The method of claim 25 wherein said mattercomprises animal products with the steps of maintaining said temperaturebetween about minus 1° and plus 2°C and maintaining said pressurebetween about 8 and 50 mm Hg.
 31. The method of claim 25 with the stepsof maintaining said hypobaric conditions for a period of time, andmodifying said conditions from time to time.
 32. The method of claim 31wherein said conditions are modified from hypobaric to cold storage.